fabric winding guidance

[braverutherford]

I've built a setup for taking a mesh-like fabric from a large supply spool and wrapping it onto smaller spools. Currently the fabric travels from the supply spool, through two closely spaced reels which are fitted with a slip clutch to provide specific tension, then directly onto the smaller spool. The fabric tends to always want to run-out on the takeup spool. I've ensured that the supply, tensioner and takeup reels are as parallel as concievably possible. Can anyone provide any guidance on a rule of thumb regarding other idler or alignment reels that i may want to incorporate into the travel of the fabric?

 

[MikeHalloran]

Web handling machinery typically includes some "dancer rolls", either very slightly convex or arranged with a flexible controlled skew mounting, or both, for precisely that purpose.



Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

You haven't provided much information. Identify where the material is shifting, on the let-off spool, on intermediate rollers, or on the wind-up spool. It is not unusual for rolls to "telescope" during rewinding operations. Understanding the physics involved is more complex than most would imagine.

Typically you want a nip roll or "s"-wrap rolls to control web speed and isolate the tension zones of unwind and rewind.
Your rolls are probably telescoping because the layers in the roll are slipping and tightening during the operation. Sometimes, active edge guiding is the only solution. Usually "taper tension" control is needed. This is a compromise between constant torque center-winding and constant tension winding. Provide some useful information for us to help.

 

[braverutherford]

Thanks MikeHalloran,
I think we'll look to incorporate another roller or two and perhaps mount one to be slightly adjustable for minor skew adjustments as you described. I am curious however regarding the rollers being slightly convex as you say. You do mean the center diameter is slightly larger than at the ends, right?

This seem unintuitive for trying to maintain alignment (but I admit to little experience in this field).

 

[braverutherford]

Thanks Compositepro,
The the "telescoping" is happening on the takeup spool. I've included a diagram illustrating our rewind fixture. Between the supply and takeup spools are a single set of tensioner rolls. The tensioner rolls are geared together and are fitted with a slip clutch to provide a constant tension at the takeup. I had hoped to avoid a complex 'taper tension' control by using the tension rolls this way.

I've included an diagram of the setup. You'll see that it's rather simple, and I fear now that it's a little too simple. I can appreciate that the physics of rewinding is more complex than most imagine.

It has been hinted to me by some that there some rules-of-thumb or conventional wisdom regarding the number or placement of idler rollers to help distribute tensions across the width of a fabric, but i can't find such clues stated anywhere.

I supposed we'll try to start adding some rollers and edge guidance to see how much improvement we can achieve.

 

[MikeHalloran]

http://en.wikipedia.org/wiki/Web-guiding_systems

Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

Since you don't say, I can only assume that you are direct driving the take-up spool, so speed will vary as roll diameter changes. What is controlling let-off tension? Your system is pretty primitive but that does not mean it cannot work. What is your material? What are your quality requirements?

You will probably find that your web is not wandering much during winding, so adding rollers will do nothing. The problem with constant tension center-winding (driving through the center of the spool) is that only the outer wrap of material is at constant tension. Winding torque must increase linearly with roll diameter to maintain tension. This increases the tension on the inner wraps after they are already on the roll. At a certain point these inner wraps will slip and tighten. They will also move laterally while doing this resulting in a telescoped roll. With some materials side flanges can be used to prevent telescoping. This can also damage the edges of the material and works better for let-offs than wind-ups.

Roll diameters do not change rapidly and it is very common for operators to change clutch settings as the diameter changes.

 

[braverutherford]

Forgive me, the uptake is driven with a hand crank. Speed is relatively slow and manually driven.

Let-off tension is provided by only a slight drag to the supply spool, which i assume to be negligible (only enough drag to prevent momentum of supply spool from dropping fabric faster than it is pulled through the tensioner reels, much much less than the tension that is provided by the tensioner reels).

The material is a woven sheet consisting of uniformly placed fibers and spacing filaments.

We want to apply tension to the sheet on uptake so that spacing filaments are not over relaxed or over stretched on uptake. The ultimate goal is that the same length of material is wound to the same outer diameter in each successive uptake run. Exact quality requirements are still being defined.

My intuition tells me that due to the nature of the material (having a 'ridged' surface due to uniformly spaced fibers) little slipping or tightening can happen to layers already on the roll, but this is a very qualitative observation that hasn't been investigated explicitly.

I do appreciate your interest in our problem and your willingness to share your insights.

 

[Compositepro]

Ridged material is particularly difficult to wind. It behaves like winding a multitude of narrow strips next to each other. The fiber will have a strong tendency to slip-off the ridges in the roll an into the groves. This will also happen inside the roll after winding and it will become a miss-shaped mess.
One solution is to add a sheet of somewhat rigid paper or plastic film into your roll to stabilize it.

Your best bet, though for your simple set-up, and type of material is to wind onto flanged spools.